With an increase in capacity of information processing in recent years, various information recording technologies have been developed. In particular, the surface recording density of an HDD using magnetic recording technology is continuously increasing at an annual rate of approximately 100%. In recent years, an information recording capacity exceeding 160 GB per one magnetic disk with a 2.5-inch diameter for use in an HDD or the like has been desired. To fulfill such demands, an information recording density exceeding 250 Gbits per one square inch is desired to be achieved.
To attain a high recording density in a magnetic disk for use in an HDD or the like, a magnetic disk of a perpendicular magnetic recording type has been suggested in recent years. In a conventional in-plane magnetic recording type, the axis of easy magnetization of a magnetic recording layer is oriented in a plane direction of a base surface. In the perpendicular magnetic recording type, by contrast, the axis of easy magnetization is adjusted so as to be oriented in a direction perpendicular to the base surface. In the perpendicular magnetic recording type, compared with the in-plane recording type, a thermal fluctuation phenomenon can be more suppressed at the time of high-density recording, and therefore the perpendicular magnetic recording type is suitable for increasing the recording density.
Conventionally, as a magnetic recording layer, CoCrPt—SiO2 or CoCrPt—TiO2 having a granular structure has been widely used. In Co, a crystal of a hcp structure (a hexagonal close-packed crystal lattice) grows in a columnar shape, and Cr and SiO2 (or TiO2) are subjected to segregation to form a non-magnetic grain boundary. By using such a granular structure, physically independent fine magnetic grains can be easily formed, and a high recording density can be easily attained.
In the above perpendicular recording type, a magnetic-monopole-type perpendicular head is used to cause a magnetic field in a direction perpendicular to the magnetic recording layer. However, with the use of a magnetic-monopole-type perpendicular head alone, a magnetic flux from a magnetic monopole immediately tries to return to a return magnetic pole on an opposite side, and therefore it is impossible to apply a sufficiently strong magnetic field to the magnetic recording layer. Thus, a soft magnetic layer is provided under a magnetic recording layer of a perpendicular magnetic recording disk to form a path (magnetic path) for a magnetic flux in the soft magnetic layer, thereby making it possible to apply a strong magnetic field in a direction perpendicular to the magnetic recording layer. That is, the soft magnetic layer is a layer in which magnetizing directions are aligned based on the magnetic field at the time of writing to dynamically form a magnetic path.
However, when a strong magnetic field is applied to the magnetic recording layer, a leakage magnetic field to an adjacent track becomes large. From this, WATE (Wide Area Track Erasure) becomes a problem, which is a phenomenon where recorded information is lost over several μm with a write-target track being taken as a center. As a technique of reducing WATE, it is said to be important to set an inverted-magnetic-domain nucleation magnetic field Hn of the magnetic recording layer as being negative and set its absolute value as being large. To obtain a high (large absolute value) Hn, a CGC (Coupled Granular Continuous) medium has been devised (Patent Document 1), in which a thin film (continuous layer) is formed that shows a high perpendicular magnetic anisotropy above or below the magnetic recording layer having a granular structure.
Also, although a high recording density can be achieved with an improvement of a coercive force Hc of the magnetic recording medium, writing with a magnetic head tends to become difficult. Thus, with an improvement in saturation magnetization Ms, the continuous layer also has a role of improving writability, that is, an overwrite characteristic.
In other words, an object of providing a continuous layer onto the magnetic recording layer is to improve the inverted-magnetic-domain nucleation magnetic field Hn to reduce noise and improve the saturation magnetization Ms to also improve the overwrite characteristic. Note that, although the continuous layer is also referred to as an auxiliary recording layer or a cap layer, it is referred to as a continuous layer in the present application unless otherwise specified.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-346315